Miniature ceramic microphone



' April 15, 1969 J. SOBEL ET L MINIATURE CERAMIC MICROPHONE Filed May 16, 1966 Inventors L %K Y w .D m J m HW A C F Y. W

y B O 3 M 2 a 3 %G I United States Patent As is well known, a microphone is a device for trans- I lating sound waves into proportional electrical signals. A microphone adapted for use in a hearing aid must, of course, be physically small yet produce suflicient output to drive an amplifier and earphone to a volume sulficient to overcome the hearing loss of a hearing aid user. Although the magnetic microphone has enjoyed the most widespread use in hearing aids utilizing transistor circuitry, the piezoelectric microphone could provide distinct advantages if incorporated into a unit of suitable size. More specifically, the piezoelectric microphone does not require the critical tolerances and the adjustment and maintenance of delicate balance that is required in a magnetic microphone, yet produces an equivalent output and is more economical to manufacture.

In the past, those concerned with the development of hearing aid microphones have found it diflicult to take full advantage of the piezoelectric microphone because, at least in part, of their inability to m'iniaturize such microphones. This has resulted principally from the lack of a practical means for supporting and maintaining electrical contact with the piezoelectric element. For example, prior attempts have generally required a separate means to support the piezoelectric element, to provide electrical contact with the various terminals on the element and to provide output terminals for coupling an amplifier or the like to the electrical contacts. The necessity of utilizing these separate components has increased the size of piezoelectric microphones to a point where their applicability to hearing aids has been severely limited.

Accordingly, an object of the present invention is the provision of a new and improved miniature piezoelectric microphone.

Another object is to provide a piezoelectric microphone having a unique ceramic bimorph element and diaphragm.

A further object of the invention is the provision of a miniature piezoelectric microphone having support means which provides simultaneous support, electrical contact, and output terminals for the transducer element.

Still another object is to provide a miniature piezoelectric microphone of the ceramic type which is characterized by simplicity of construction, low cost and high reliability.

A miniaure microphone in accordance with the present invention comprises a housing having a sound wave admitting aperture and a diaphragm peripherally secured to the housing adjacent the aperture. A bimorph element is secured intermediate its ends to the diaphragm and is comprised of a pair of oppositely polarized ceramic wafers having first and second electrodes on each of the respective outer faces of the wafers and a third electrode between and contacting both of the inner faces of the wafers. In addition, a pair of electrically conductive supports are disposed at the ends of the element for supporting the latter, one of the supports electrically contacting the first and second electrodes but being insulated from the third electrode and the other of the supports "ice electrically contacting the third electrode but being insulated from the first and second electrodes, both of the supports having output terminals formed thereon and extending through the housing.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of op eration of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIGURE l is a plan view of a preferred embodiment of the invention, partly broken away and partly in section, taken along the lines 1 1 of FIGURE 2;

FIGURE 2 is a side elevation view of the device shown in FIGURE 1, partly in section, taken along the lines 22 of FIGURE 1; and

FIGURE 3 is an end view of the device shown in FIG- URE 1, partly in section, taken along the lines 3-3 of FIGURE 1.

Referring now to the drawings, there is shown a microphone 10 having a housing 12 which comprises a top portion 14 and a bottom portion 16. The top portion 14 is recessed as at 18 around its periphery and is provided with depending flanges 20 secured to the side walls of bottom portion 16. A sound wave admitting aperture 22 is formed in a recessed central portion 24 of the top portion 14 and is covered with a cylindrical porous metal disc 26 which serves as an acoustic filter and controls the amplitude of the resonant frequency of the microphone 10 by damping the sound wave peaks. It has been found that the use of a porous metal disc provides more uniform damping than conventional damping cloth since the latter is subject to denier variations from lot to lot whereas the porosity of the former is more easily controllable and uniform. The disc 26 may be formed of any suit-able material, such as steel, by a conventional sintering process.

A diaphragm 28 is clamped at its periphery 29 between the upper and lower housing portions 14 and 16, respectively, and is provided with a sealing gasket 30 conforming to the shape of housing 12 disposed between the upper surface of the diaphragm 28 and" the top portion 14. The diaphragm 28 is constructed of a flexible plastic material firmly bonded between a pair of metal sheets to form a tri-layer laminate. In a preferred construction of the diaphragm 28 the flexible plastic material is formed of polyethylene terephthalate, sold under the trademark of Mylar, while the upper metal sheet is formed of aluminum and the lower metal sheet is formed of brass. The diaphragm 28 has a generally rectangular outer configuration to conform to the shape of the housing 12 and, as best illustrated in FIGURE 1, is provided with an oval depression 32 near its center which is circumscribed by a compliant rib 34 of exposed Mylar. This construction provides the necessary axial compliance at the edges of the diaphragm to provide high sensitivity while maintaining radial and circumferential rigidity at the center of the diaphragm to permit a piston like motion thereof. The diaphragm 28 is secured at its center by a suitable bonding agent, such as an epoxy cement, to a drive pin 36 which in turn is secured at its base 38 to a bimorph ceramic element 40.

The bimorph element 40 is comprised of a metal shim 42 interleaved between a pair of piezoelectric ceramic wafers 44, 44, each of the latter having an electrically conductive coating 46, 46 bonded to its exposed surface. The bimorph element 40 is comprised, therefore, of a pair of piezoelectric ceramic wafers 44, 44 having first and second outer electrodes 46, 46' and a third inner electrode 42. Since the ceramic wafers 44, 44' are flexed in opposite directions by the action of the diaphragm 28, they must be oppositely polarized so that the signals developed by each will be additive. An example of an element which has been found to be particularly effective is one constructed of an aluminum shim, a pair of oppositely polarized lead zirconium titanate ceramic wafers and a silver frit conductive coating. Of course, other materials may be substituted depending on the particular application and the desired characteristics of the microphone.

The bimorph element 40 is supported by a pair of electrically conductive metallic spring clips 48, 48' having resilient C-shaped jaws 50, 50' formed at one end and output terminals 52, 52 formed at the other end. The spring clips, 48, 48' support the element 40, make electrical contact with the electrodes 42, 46 and 46 and provide output terminals therefor. More specifically, as best illustrated in FIGURE 2, the spring clip 48 electrically contacts the first and second electrodes 46 and 46' through an electrically conductive U-shaped strip 54, formed, for example, of a suitable metal foil, which is wrapped around the end of the transducer 40 and respectively contacts the electrodes 46, 46' through a pair of legs 55, 55 and the jaw 50 of the spring clip 48 through the leg 55'. A shock absorbing pad 56, of rubber or the like, is interposed between the lower portion of the C- shaped jaw 50 and the leg 55 of the strip 54 for isolating the element 40 from mechanical shock. The second spring clip 48' electrically contacts the third electrode 42 through a second U-shaped metallic foil strip 58, one leg 59 of the strip 58 contacting the C-shaped jaw 50' of the spring clip 48' and the other leg 59 contacting the third electrode 42. The leg 59 of the strip 58 may be inserted between the ceramic wafer 44 and the third electrode 42 at any convenient stage in the manufacture of the element 40. The

leg 59' of the conductive strip 58 is electrically insulated from the major portion of the electrode 46 by a break 60 in the electrode 46 adjacent one end of the element 40. A shock absorbing pad 62 is utilized with the spring clip 48' in a manner similar to that of pad 56 but further insulates the spring clip 48' from the electrode 46. A pair of electrical conductors 64 are secured, as by soldering, to the output terminals 52, 52 formed respectively at the ends of the spring clips 48, 48', the latter being secured, by a suitable cement, to a plastic block *66 disposed within the lower portion 16 of the housing 12. The output terminals 52, 52' extend through an aperture 57 formed in the bottom portion 16 of the housing 12 so that the conductors 64 may be easily connected and disconnected without separating the housing portions.

The microphone 10 is provided with a Thuras or vent tube 68 having an enlarged upper flange 70 protruding through the upper portion 14 of the housing 12 and communicatin-g with the atmosphere, and a lower opening 72 terminating within the lower portion 16 of the housing 12. The Thuras tube 68 is formed or cut to a predetermined length to tune the frequency response of the microphone 10, in a manner well known in the art. In a preferred embodiment of the invention, the Thuras tube 68 is designed to boost the frequency response in the 350 to 400 cycle per second range and provide a gradual drop thereafter.

In operation, sound waves passing through the porous metal disc 26 and the aperture 22in the top portion 14 of the housing 12 strike the diaphragm 28 and cause the latter to vibrate. These vibrations are transmitted to the bimorph element by the drive pin connection 36 and cause the ceramic wafers 44, 44 to flex in opposite directions and produce a voltage. Since the ceramic wafers are oppositely polarized and are electrically connected in parallel by the three electrodes 42, 46 and 46', the voltages produced by each are additive and appear at the output terminals 52, 52 in proportion to the intensity and frequency of the sound waves striking the diaphragm 28. The conductors 64 may be connected in a conventional manner to a suitable amplifier and earphone for reproducing the original sounds. Although the microphone 10 may be used for a variety of purposes, its small size, as more fully set forth above, makes it particularly suitable for use in hearing aids.

There has thus been described a miniature microphone having an improved ceramic piezoelectric element and improved support means therefor, the latter providing simultaneous support, electrical contact and output terminals for the element. The support means, by combining these separate functions into a single support member, simplifies the construction, lowers the cost, and decreases the physical size of the microphone while maintaining a high degree of reliability.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A miniature microphone for use in hearing aids and the like comprising:

a housing having a sound wave admitting aperture;

a diaphragm peripherally secured to said housing adjacent the aperture;

a bimorph element secured intermediate its ends to said diaphragm, said element comprising a pair of oppositely polarized ceramic wafers having first and second electrodes on each of the respective outer faces of said wafers and a third electrode between and contacting both of the inner faces of said wafers; and

a pair of electrically conductive supports disposed at the ends of said element for supporting the latter, one of said supports electrically contacting said first and second electrodes and insulated from the third electrode and the other of said supports electrically contacting said third electrode and insulated from the first and second electrodes, said supports having a pair of output terminals formed thereon and extending through said housing.

2. A microphone in accordance with claim 1 wherein:

said electrically conductive supports comprise a pair of metallic spring clips, each of said clips having a C-shaped resilient jaw at one end for gripping the ends of said element.

3. A microphone in accordance with claim 2 wherein:

said housing is formed of a top and bottom portion,

said diaphragm being clamped at its. periphery between said top and bottom portions, said diaphragm comprising a flexible plastic material bonded between a pair of metal sheets and having a circumferential compliant rib of exposed plastic material, whereby said diaphragm is compliant in an axial direction and rigid in a radial direction.

4. A miniature piezoelectric ceramic microphone for use in hearing aids and the like comprising:

a housing having top and bottom portions, the top portion having a centrally disposed aperture;

a diaphragm clamped at its periphery between the top and bottom portions of said housing, said diaphragm comprising a sheet of polyethylene terephthalate bonded between a sheet of aluminum and a sheet of brass, the polyethylene terephthalate sheet being exposed near the periphery of the diaphragm, said diaphragm having a central axis substantially coincident with the central axis of the aperture;

a porous metal disc secured to said top portion over the aperture;

a vent tube communicating the bottom portion of said housing to the atmosphere, said vent tube being of a predetermined length for tuning the response of said microphone;

a multilayer bimorph element connected at its center to said diaphragm, said element comprising a metal shim having a pair of piezoelectric ceramic wafers secured to the opposite sides thereof, each of said ceramic wafers having an electrically conductive coating on its exposed surface; and

a spring clip disposed at each end of said element for providing support, electrical contact and electrical output terminals for said element, one of said spring clips electrically contacting said metal shim and being insulated from said electrically conductive coatings and the other of said spring clips electrically contacting both of said electrically conductive coatings and being insulated from said metal shim, said spring clips being secured to the bottom portion of said housing and terminating in a pair of output terminals extending through said housing.

References Cited 5 UNITED STATES PATENTS 1,649,081 11/1927 Rudolph. 2,411,146 11/1946 Clement.

KATHLEEN H. CLAFFY, Primary Examiner. ARTHUR A. McGILL, Assistant Examiner.

US. Cl. X.R. 

1. A MINIATURE MICROPHONE FOR USE IN HEARING AIDS AND THE LIKE COMPRISING: A HOUSING HAVING A SOUND WAVE ADMITTING APERTURE; A DIAPHRAGM PERIPHERALLY SECURED TO SAID HOUSING ADJACENT THE APERTURE; A BIMORPH ELEMENT SECURED INTERMEDIATE ITS ENDS TO SAID DIAPHRAGM, SAID ELEMENT COMPRISING A PAIR OF OPPOSITELY POLARIZED CERAMIC WAFERS HAVING FIRST AND SECOND ELECTRODES ON EACH OF THE RESPECTIVE OUTER FACES OF SAID WAFERS AND A THIRD ELECTRODE BETWEEN AND CONTACTING BOTH OF THE INNER FACES OF SAID WAFERS; AND A PAIR OF ELECTRICALLY CONDUCTIVE SUPPORTS DISPOSED AT THE ENDS OF SAID ELEMENT FOR SUPPORTING THE LATTER, ONE OF SAID SUPPORTS ELECTRICALLY CONTACTING SAID FIRST AND SECOND ELECTRODES AND INSULATED FROM THE THIRD ELECTRODE AND THE OTHER OF SAID SUPPORTS ELECTRICALLY CONTACTING SAID THIRD ELECTRODE AND INSULATED FROM THE FIRST AND SECOND ELECTRODES, SAID SUPPORTS HAVING A PAIR OF OUTPUT TERMINALS FORMED THEREON AND EXTENDING THROUGH SAID HOUSING. 